Hello Alex,
thanks for your response. The flow should be pretty standard for this kind of implementation.
First of all user should log in in standard way, providing login and password. Next, if provided data are correct, display form with some information and input box for code should appear. The code will be send by SMS. The combination of login/password and code from SMS will login into portal.
The engine for SMS send and verification is on our customer site.
We don't have mock-up of a login page for now.
Thanks,
Bartosz